1. Field of the Invention
The present invention relates to a sheet processing apparatus that aligns and binds sheet-shaped members (simply referred to as “sheets” in the specification) such as sheets of paper, recording sheets, transfer sheets, or OHP sheets that are carried in, an image forming system that includes the sheet processing apparatus and an image forming apparatus such as a copying machine, a printer, a facsimile, or a digital multi-function peripheral (MFP), and a sheet processing method which is executed in the sheet processing apparatus.
2. Description of the Related Art
Conventionally, an apparatus called a finisher that includes a stapler is known; the finisher allows sheets discharged from an image forming apparatus to be stacked on a stapler tray, aligns the stacked sheets in a sheet-conveying direction (a so-called longitudinal direction) and a direction perpendicular to the sheet-conveying direction (a so-called width direction), and binds the aligned sheets together. In a case where end-face-binding of an end face is performed by using a stapler, the stapler moves in the direction perpendicular to the sheet conveying direction along a sheet edge portion (usually, the trailing end of a sheet) abutting on a reference fence that defines the position of the sheet in the conveying direction, so that the binding position can be changed. Then, in order to improve the alignment precision of the finishing of a bundle of bound sheets, the posture of the trailing end of the sheets stacked on the staple tray may be maintained. Thus, there is a known configuration in which a sheet bundle is pressed in a state in which the trailing end of the sheet bundle is abutting on the reference fence. Such a configuration includes a pressing member (trailing end pressing lever) 110 illustrated in FIG. 1 to be described later, and the pressing member 110 is provided at a lower end portion of a trailing-end reference fence 51 so as to press the trailing end of the sheet bundle SB housed in the trailing-end reference fence 51 and is configured to be capable of reciprocating in a direction approximately perpendicular to an end-face-binding processing tray F (see FIG. 7 to be described later).
However, in the sheet maintaining configuration of the conventional end binding process, and more specifically, in a configuration in which the posture of the sheet bundle (in particular, the trailing end thereof) is maintained inside the staple tray, a pressing member for pressing and maintaining the sheet bundle presses a position distant from the stapler in consideration of the interference thereof with the stapler. Accordingly, bending or the like occurs on the sheet bundle when the sheets are bound with a staple, making it difficult to achieve precise alignment. In addition, because the pressing member is fixed in the width direction of the sheet bundle, the pressing member sometimes presses a position located more distant from the position of the staple depending on the sheet size or the binding position.
Thus, for example, Japanese Patent Application Laid-open No. 2008-19028 discloses a configuration which includes a pressing member provided at a position near a reference fence and a control unit that changes at least one of the number of times and the duration for pressing sheets by using the pressing member for the purpose of preventing binding from failing.
However, even when the pressing member is arranged at a position near the reference fence, and at least one of the number of times and the duration for pressing the sheets by using the pressing member is changed similarly to the invention disclosed in Japanese Patent Application Laid-open No. 2008-19028, the pressing position is not changed in accordance with the sheet size or the binding position. Accordingly, there are cases where the pressing position and the binding position are separated from each other. In such cases, the bending as described above may occur, and it is difficult to sufficiently cope with the change in the pressing position or the sheet size.
Thus, there is a need to achieve high precision in aligning the sheet bundle regardless of the sheet size or the binding position.